1. Field of the Invention
The present invention relates to a heating device using an electromagnetic induction heating method, a fixing device of an image forming apparatus including the heating device, and an image forming apparatus including the fixing device, such as a copier, a printer, a facsimile machine, and a complex machine thereof.
2. Discussion of the Background Arts
In a widely-known background image forming apparatus, such as a copier and a printer, a fixing device using an electromagnetic induction heating method is used to reduce the start-up time of the fixing device in order to reduce energy consumption.
A first example of the background fixing device using an electromagnetic induction heating method includes a support roller (i.e., a heating roller), a fixing support roller (i.e., a fixing roller), a fixing belt extended under tension by the support roller and the fixing support roller, an induction heating device facing the support roller via the fixing belt, a pressure roller in contact with the fixing support roller via the fixing belt, and so forth. The induction heating device includes an exciting coil extending in the width direction (i.e., a direction perpendicular to a direction of conveying a recording medium), an exciting coil core facing the exciting coil, and so forth.
The fixing belt is heated at a position facing the induction heating device. The heated fixing belt heats a toner image formed on the recording medium, when the recording medium is conveyed to a position between the fixing support roller and the pressure roller. Thereby, the toner image is fixed on the recording medium. Specifically, the exciting coil is applied with a high-frequency alternating current, and an alternating magnetic field is generated around the exciting coil. Thereby, an overcurrent is generated near a surface of the support roller, and Joule heat is generated due to the electric resistance of the support roller. The thus generated Joule heat is used to heat the fixing belt wound around the support roller.
In the first example of the background fixing device using an electromagnetic induction heating method, the surface temperature of the fixing belt (i.e., a fixing temperature) can be increased to a desired value in a relatively short start-up time with lower energy consumption than in a fixing device using another method, such as a heat roller method.
In a second example of the background fixing device using an electromagnetic induction heating method, a magnetic conductor having a Curie point is used to form a heat-generating member (i.e., a heat-generating device) so that the heat-generating member has a self-temperature controlling function.
Further, in the second example of the background fixing device, to prevent the heat-up time at start-up of the fixing device (i.e., the start-up time) from being prolonged, two magnetic metal members of different Curie points are laminated with each other to form a heat-generating member, and the frequency of the alternating current supplied to an exciting member (i.e., an exciting device) is changed.
In a third example of the background fixing device using an electromagnetic induction heating method, the core of an induction heating device sandwiches a fixing belt. That is, the core of the induction heating device faces both the outer circumferential surface and the inner circumferential surface of the fixing belt for improving the heat-generating efficiency of the fixing belt.
In the second example of the background fixing device, the heat-generating member has the self-temperature controlling function. Therefore, the temperature of a fixing member can be prevented from being excessively increased, with no need for a complicated temperature controlling operation performed by an electric circuit, compared with the first example of the background fixing device. However, the second example of the background fixing device is still open to improvements.
For example, at a temperature near the Curie point of the heat-generating member, the relative magnetic permeability of the heat-generating member (i.e., a conductive layer thereof) is decreased, and the gradient of the increase in temperature of the heat-generating member is reduced. Due to the reduction of the gradient of the temperature increase, therefore, it is highly possible that the heat-up time at start-up of the fixing device is prolonged. That is, in the second example of the background fixing device, while the excessive increase in temperature of the fixing member can be prevented by the self-temperature control of the heat-generating member, the start-up performance of the fixing device is insufficient.
In view of the above, if the two magnetic metal members of different Curie points are laminated to each other to form a heat-generating member, and if the frequency of the alternating current supplied to the exciting member is changed, an effect of preventing the excessive increase in temperature of the fixing member through the self-temperature control of the heat-generating member, and an effect of reducing the start-up time of the fixing device can be expected. That is, through the adjustment of the frequency of the alternating current, the magnetic metal member having a Curie point higher than the fixing temperature is caused to generate heat at the start-up of the fixing device for preventing the reduction of the gradient of the temperature increase. Then, after the fixing device has been started up, the other magnetic metal member having a Curie point near the fixing temperature is caused to generate heat for preventing the excessive increase in temperature of the heat-generating member.
In the above example of the background fixing device, the benefits of both a reduction of the start-up time and a prevention of an excessive increase in temperature can be obtained. The background fixing device, however, tends to be complicated in structure due to the multilayer structure of the heat-generating member and to be relatively high in cost. Further, since the heat-generating member has the multilayer structure including the two magnetic metal members of different linear expansion coefficients, it is highly possible that heat strain occurs in the heat-generating member due to the difference in the linear expansion coefficients. If heat strain occurs in the heat-generating member, the layers of the heat-generating member may be separated and damaged, and thus the performance of the heat-generating member (e.g., conveying performance of a fixing belt formed by the heat-generating member) may be deteriorated.
On the other hand, in the third example of the background fixing device, the shape of the core of the induction heating device is improved such that the core sandwiches the fixing belt to improve the heat-generating efficiency of the fixing belt. However, prevention of an excessive increase in temperature of the fixing member cannot be obtained.
The above-described limitations are also found in other conventional devices.